3GPP's LTE standard uses Orthogonal Frequency Division Multiple Access as the primary multiple access scheme for the downlink. Noise and interference levels need to be taken into account at the receiver for optimum performance. As cellular networks get loaded with users, interference from other cells becomes the most dominant source of interference. Since the other cells employ the same standard, this interference is likely to be highly frequency selective. Thus, frequency selective interference estimation and inclusion of the estimates in the soft-metrics for the forward error correction is likely to increase the receiver performance. Unfortunately, as frequency selective interference can be estimated less accurately, the receiver performs worse in noise limited scenarios than a receiver that only takes the average interference level into account.
Maximum likelihood decoding is an approach to achieve the best possible performance by identifying the sequence of presumably transmitted information from a received signal that has been sent using a maximum likelihood (ML) approach. The theory is well known (see e.g. J. G. Proakis, Digital Communications, 2nd ed., McGraw-Hill, 1989). Turbo and Viterbi decoders provide an ML estimate for received turbo encoded code blocks and convolutional encoded code blocks, respectively, provided that the encoded sequence include the correct soft-metrics. A soft-metric represents the likelihood that the received bit is correct. The soft-metrics can be related to the signal-to-noise ratio for that bit.
In wireless communications, where the mobile channel is characterized by time and frequency selective fading, the signal strength can vary substantially over time and frequency. As thermal noise is additive to the received signal at the receiver antennas and the thermal noise is typically wideband, the thermal noise level can be assumed constant for each received symbol.
As cellular communication systems get loaded with multiple users, cellular communication systems become interference limited rather than noise limited. Interference limitation means that the mobile receiver's performance is not limited by the signal strength with respect to the thermal noise level, but by the interference originating from other users or base stations in the network operating at the same frequency. Noise limitation is only relevant for the so called network edges, that is, those cell edges where the mobile has to operate at maximum sensitivity level.
The new cellular standard developed by the 3rd generation partnership program (3GPP) called Long Term Evolution (LTE) offers unprecedented data rates and unprecedented shortest latency to the end customer while at the same time promising a high spectral efficiency for the network operator. This allows network operator to make best use of the available spectrum.
The LTE standard separates a channel into time and frequency blocks that can be allocated to a specific mobile terminal in the downlink, i.e., in transmissions from base station to a terminal. The time granularity is one slot that has a fixed duration of 0.5 ms. Orthogonal frequency division multiple access (OFDMA) is used to distribute the available downlink bandwidth to multiple users. In OFDMA, all user information is modulated using Orthogonal Frequency Division Multiplexing (OFDM), where each user is assigned a different set of subcarriers.
FIG. 1 illustrates how OFDMA works. Here, the available subcarriers of the downlink signal are given to three different users. LTE makes assignment to different users, by overlaying a grid over the time-frequency plane. This grid is referred to as the time-frequency grid. The smallest block addressable by the system is called a resource block (RB). It consists of 12 OFDM subcarriers over one time-slot. RBs are always allocated to a user in a pair of two successive RBs. FIG. 2 illustrates the concept of the time-frequency grid and RBs for a channel where only 72 subcarriers are used. Such a channel fits into a 1.6 MHz channel and uses the minimum defined bandwidth in LTE.
An RB always consists of 12 subcarriers and has a duration of 0.5 ms which are 6 or 7 OFDM symbols depending on the mode the base station uses.
As the network gets loaded, as described above, users get more exposed to signals received from adjacent cells operating at the same frequency (co-channel) than to thermal noise. FIG. 3 shows an illustrative example of co-channel interference. Terminal UE1 (UE—User Equipment) not only receives a signal from the left hand cell served by base station 1 (BS1) but is also exposed to the right hand cell served by BS2. Thus, UE1 receives the signal of BS2 intended for UE2 as interference.
As the origin of the interference is also an LTE base station transmitter, its characteristics are similar to the wanted signal:    (1) The interfering base station (BS2) assigns different RBs to different users, with different beam forming patterns and power levels. RB allocations vary from one sub-frame to the next.    (2) The interfering signals received by UE1 from BS2 are received via a mobile communications channel.Thus, the interference is highly time and frequency-selective.
In a real system, the communication channel and the noise and interference levels are never known, but need to be estimated.
Assuming that the co-channel interference can be treated as random noise, the optimum receiver not only needs to estimate the channel for the signal-of-interest, but also the interference level for each resource element individually. While the channel can be estimated with rather reliability using reference symbols that are multiplexed into the OFDMA signal, the interference level is estimated less precisely, especially if a high time-frequency resolution of the interference level is desired.
Analysis have shown, however, that a receiver that estimates frequency selective interference and considers these estimates in the soft-metric calculation can outperform a conventional receiver that treats the interference as a constant over the entire frequency, by several decibels when the receiver is exposed to a high level of frequency selective interference.
However, in case co-channel interference is not present, e.g., in a thermal noise limited reception scenario, the receiver performance suffers from the estimation error.
Special performance tests as proposed by the standardization body (3GPP) and adopted by the Global Certification Forum (GCF) perform most of the performance related tests without frequency selective interference. Albeit expected to outperform the conventional receiver in the field, the receiver that makes use of the frequency selective interference soft-metric calculation technique exhibits inferior performance in the test environment.
A general object of the invention is to increase LTE receiver performance in loaded networks without impacting receiver performance in unloaded networks.